Electronic keying circuit



1947. F; M. BERRY ETAL 1 2,425,667

ELECTRONIC KEYING CIRCUIT Filed June 25, 1945' 1 TRANSMIT ER KEYING STAGE GRD CIRCUIT IIIIIFI+IIW MIIZI fieliJafiagg/w taxman; )Z. eww,

A W WW KEYED CARRIER CURRENT snURcE Patented Aug. 12, 1947 ELECTRONIC KEYING CIRCUIT Fred M. Berry, Kansas City, and William R. Lewis,

Independence, Mo.

Application June 25, 1945, Serial No. 601,366

Claims. 1

The present invention relates to an electronic keying circuit and more particularly to a so-called carrier-current keying system by means of which the keyed stage of a radio telegraph transmitter may be actuated and controlled from a remote location through the medium of telephone lines or the like.

A radio telegraph transmitter can be satisfactorily keyed (that is turned on and off) at rates up to two hundred cycles per second by varying the direct-current potential applied to a grid circuit in one of its low level stages. Generally speaking there are no problems involved in keying a transmitter directly from a short distance by making and breaking a direct-current circuit applying a variable to zero direct-current potential to the control grid in the keyed stage of the transmitter. When it is desired, however, to key the transmitter from a remote location through the medium of a telephone line it is not practical to use a direct-current circuit since the direct-current cannot be amplified or passed by the repeating coils customarily used in the telephone line. Therefore, remote keying of a radio telegraph transmitter is usually done by connecting to the input of a telephone line through a sending key an alternating current of audio frequency which may be amplified and passed by the telephone line and its repeater coils to the transmitter location. At the transmitter, the keyed audio frequency is rectified or converted to direct-current and applied to the grid circuit of the keyed stage of the transmitter to thus transmit intelligence by keyed radio frequency waves. In order that the transmitter may be keyed at relatively high speed suchas keying rates up to two hundred cycles per second, it is necessary that the direct current applied to the grid circuit attain a value that will key the transmitter in a very short period of time. In

- other words the direct-current voltage that is to control the transmitter must vary in the same manner as the key impulses at the remote location and must not be distorted or lagged in the speed of its responsive variations due to the intermediate steps of converting the alternating current transmission to direct current. It is also desirable that the residual alternating current ripple voltage be completely filtered and removed from the rectified direct current applied to the transmitter so as to avoid any tendency of the transmitter to be keyed by such alternating current. A filter circuit for such purposes must have a very short time constant in order not to introduce a delay in the responsiveness of the key-v 2 ing circuit and the keying speed of the transmitter.

It is a principal object of this invention to provide a radio telegraph keying circuit responsive to the impulses received from a keyed carrier current transmitted over a telephone line to produce a filtered direct current voltage varied towards zero in accordance with the impulses and which maybe applied as a keying voltage to a control grid in the keyed circuit of the transmitter.

It is also an important object of this inven tion to provide an electronic keyingcircuit that will convert a keyed carried current into a keyed or varyin direct current voltage by means of a rectifier and a very simple R. C. filter with a Very short time constant of second or less and which eliminates any residual alternating current ripple voltage by means of electronic tubes.

Further objects and advantages of the invention will be apparent by reference to the following specification and drawings in which,

The single figure is a schematic diagram of the keying circuit of this invention.

Referring to the drawing a source of keyed carrier current which may be alternating current of the frequency of 1000 cycles per second is shown in block outline by numeral ID. It should be understood that the source of keyed carrier current would include a remote sending key which is used to make and break the circuit for transmission of the audio frequency carrier current over a telephone line to the keying circuit of this invention much in the manner in which a direct current circuit is interrupted or keyed for the transmission of intelligence. The primary ll of transformer I2 is connected to the keyed carrier current source Ill. The secondary l3 of transformer I2 is center-tapped at M and connected to the double-diode tu be l5 to constitute a full wave rectification circuit [6. It should be understood that any form of rectifier circuit using other tube types such as a triode may be used in place of the dual diode shown. The rectified voltage output of the rectification circuit appears across the load resistance l1 and filter condenser I8. The rectified voltage output appearing across the load resistance I1 is in the form of a pulsating direct current since it is impossible to remove the residual alternating current ripple voltage by means of a simple R. C. filter such as is comprised of the resistor 11 and condenser I8. The values of R and C are preferably so selected that the filter time constant is such that the voltage across the condenser I8 is discharged to not less than one-half the peak A. C. voltage before it is recharged by a succeeding peak; yet should the alternating current input to the rectifier circuit I6 stop, the condenser would be completely discharged in a time corresponding to less than one alternating current cycle. Therefore a pulsating direct current voltage is developed across resistor that does not drop below one-half the peak value of the A. C. current applied to the rectifier as long as the carrier current is uninterrupted. Should the carrier current be interrupted by a keying action the rectifier voltage appearing across the resistor I1 and condenser l8 would be removed in less than of a second in the case of a 1000 cycle per second carrier current. Thus the variation to zero in direct current voltage appearing across resistor H to be applied to the control circuit of a radio telegraph transmitter follows the original key variation with sufiicient speed to permit the keying by remote control up to rates of 200 or more cycles per second.

The rectified voltage appearing across resistor H as before stated contains alternating current ripple voltages which in the case of a frequency of 1000 cycles per second keyed carrier current could be termed an audio ripple voltage. This audio ripple voltage must be removed before the rectified direct current can be used to key a low level transmitter stage since such transmitter stage might respond to the variations in direct current due to the audio ripple component in addition to the variation to zero of the direct current. By this invention the pulsating direct current appearing across the resistor I1 is not used to directly control the keying circuit of a transmitter but instead is used to control the conductivity of an electron tube having a load resistance across which the variation of direct-current potential for controlling the transmitter may be obtained.

When there is no applied carrier current and hence no pulsating direct current across resistor H, the potential of the plates 2| of diode 20 is more positive than the potential of the cathodes 22 with the battery 23 connected as shown. The pulsating direct current which appears across the resistor when a carrier current is applied to transformer I2 is of opposing polarity to the polarity of battery 23 and hence the plates 2| are not more positive than the cathodes 22 when a carrier current is applied to the input of the rectifier circuit. Thus the diode 20 conducts during the intervals when the carrier current is interrupted and is non-conducting when the carrier current is applied. The potential of battery 23 or its equivalent potential ource is equal to not more than the minimum value of opposing pulsating direct current potential so that the rise and fall of the pulsating direct current from minimum to maximum potential values due to the audio ripple voltage does not cause a change in the non-conducting state of diode 20. Hence a voltage drop appears across the cathode load resistor 24 when diode 20 i conducting and the carrier current is interrupted. In place of the dual diode 20, a single diode, or other tube type when connected as a rectifier may be used as should be well understood. This direct current voltage drop is entirely devoid of any audio ripple voltage in view of the circuit arrangement, just referred to and it will be removed when a pulsating direct current appears across resistor due to the reconnection of the carrier Qurrent at the 4 remote keying station and the ensuing non-conducting state of diode 2|].

The voltage drop across resistor 24 is applied to the grid 25 of a relay tube, which may be the triode 26, when cathode 21 is normally more positive than grid 25 in view of the bias battery 28. When diode 20 conducts, the voltage drop across resistor 24 makes the grid 25 more positive and accordingly the triode 26 more conductive. Thus a larger current is drawn through the plate 29 producing an increased voltage drop across the plate load resistance 30. With the positive terminal of the battery 3| supplying the plate voltage for triode 26 grounded as shown, the point 32 Will become more negative with respect to point 33 a the triode 26 becomes more conductive. This variation in direct current which is devoid of audio ripple may be used in the grid circuit of a low level transmitter stage 34 to key the transmitter.

If desired, the potential of the bias battery 28 may be adjusted to a value that will render the triode 26 completely non-conductive when the diode 20 is non-conductive so that the potential of point 32 with respect to point 33 will be zero. Thus the direct current appearing across points 32 and 33 will be varied to zero in response to the keyed variations of the audio current at the source To summarize, the keying circuit of this invention it will be understood that a carrier current applied to transformer l2 produces a pulsating direct current across the load resistance I! of the rectifier circuit l6. With the simple R. C. network comprised of resistor l1 and condenser l8 a fast time constant is obtained so that with a carrier current frequency of 1000 cycles per second the voltage across resistor will be removed within /1000 of a second after the interruption of the carrier current due to a keying action. Such pulsating direct current as appears across resistor causes tube 20 to be non-conductive and the circuit is such that the tube is completely non-conductive for any value of the pulsating direct current from maximum to minimum. When tube 20 is non-conductin triode 26 is relatively non-conducting and the potential difference between points 32 and 33 is less. When the carrier current is interrupted by breaking the keying circuit, tube 20 becomes conductive and increases the conductivity of triode 29 so that the potential difierence across points 32 and 33 increases. This change in direct current potential across points 32 and 33 faithfully follows the keying of the carrier current with such rapidity as to enable high speed keying rates of a transmitter up to two hundred or more cycles per second.

It should be understood that filament heating means XX are provided for all of the tubes and that any source of direct current potential may be substituted for the batteries 23, 28 and 3|.

We claim:

1. A transmitter keying circuit responsive to the keyed impulses of alternating current applied to the input of a rectifier circuit, a resistor condenser filter circuit connected to the output of said rectifier circuit to provide a pulsating direct current proportional to the alternating current input to said rectifier, an electron tube having a cathode load resistor and a source of direct current potential connected in series with the pulsating direct current output of said filter circuit, the polarity of said direct current source opposing the polarity of said pulsating direct current, the potential value of said, direct current source being equal to not more than the minimum value of said pulsating direct current whereby the electron tube will be rendered non-conductive when there is a pulsating direct current at the output of said filter circuit and will be conductive when there is no pulsating direct current, and relay means actuated by the voltage drop across said cathode load resistor for keying the transmitter.

2. A transmitter keying circuit responsive to the keyed impulses of alternating current applied to the input of a rectifier circuit, a resistor condenser filter circuit connected to the output of said rectifier circuit to provide a pulsating direct current proportional to the alternating current input to said rectifier, said filter circuit having a time constant to provide complete discharge of said condenser within the time of one cycle of the alternating current frequency after the alternating current input has been interrupted, an electron tube having a cathode load resistor and a source of direct current potential connected in series with the pulsating direct current output of said filter circuit, the polarity of said direct current source opposing the polarity of said pulsating direct current, the potential value of said direct current source being equal to not more than the minimum value of said pulsating direct current whereby the electron tube will be rendered non-conductive when there is a pulsating direct current at the output of said filter circuit and will be conductive when there is no pulsating direct current, and relay means actuated by the voltage drop across said cathode load resistor for keying the transmitter.

3. A transmitter keying circuit responsive to the keyed impulses of alternating current applied to the input of a rectifier circuit, a resistor condenser filter circuit connected to the output of said rectifier circuit to provide 9, pulsating direct current proportional to the alternating current input to said rectifier, said filter circuit having a, time constant to provide complete discharge of said condenser Within the time of on cycle of the alternating current frequency after the alternating current input has been interrupted, an electron tube having a cathode load resistor and a source of direct current potential connected in series with the pulsating direct current output of said filter circuit, the polarity of said direct current source opposing the polarity of said pulsating direct current whereby the electron tub will be rendered non-conductive when there is a pulsating direct current at the output of said filter circuit and will be conductive when there is no pulsating direct current, and relay means actuated by the voltage drop across said cathode load resistor for keying the transmitter, said relay means including a vacuum tube having a control grid circuit and a plate circuit, the voltage drop across said cathode load resistor being applied to said grid circuit, a load resistor connected in said plate circuit, a potential supply for said plate circuit, the positive terminal of said potential supply being grounded and the voltage drop across said plate load resistor connected to the grid circuit of the transmitter keying stage.

4. A transmitter keying circuit responsive to the keyed impulses of alternating current applied to the input of a rectifier circuit, a resistor condenser filter circuit connected to the output of said rectifier circuit to provide a pulsating direct current proportional to the alternating current input to said rectifier, an electron tube having a cathode load resistor and a source of direct current potential connected in series with the pulsating direct current output of said filter circuit, the polarity of said direct current source opposing the polarity of said pulsating direct current, the potential value of said direct current source being equal to not more than the minimum value of said pulsating direct current whereby the electron tube Will be rendered non-conductive when there is a pulsating direct current at the output of said filter circuit and will be conductive when there is no pulsating direct current, and relay means actuated by the voltage drop across said cathode load resistor for keying the transmitter, said relay means including a vacuum tube having a control grid circuit and a plate circuit, the voltage drop across said cathode load resistor being applied to said grid circuit, a load resistor connected in said plate circuit, a potential supply for said plate circuit, the positive terminal of said potential supply being grounded and the Voltage drop across said plate load resistor connected to the grid circuit of the transmitter keying stage.

5. A transmitter keying circuit responsive to the keyed impulses of alternating current applied to the input of a rectifier circuit, a resistor condenser filter circuit connected to the output of said rectifier circuit to provide a pulsating direct current proportional to the alternating current input to said rectifier, said filter circuit having a time constant to provide complete discharge of said condenser Within the time of one cycle of the alternating current frequency after the alternating current input has been interrupted, an electron tube having a cathode load resistor and a source of direct current potential connected in series with the pulsating direct current output of said filter circuit, the polarity of said direct current source opposing the polarity of said pulsating direct current, the potential value of said direct current source being equal to not more than the minimum value of said pulsating direct current whereby the electron tube will be rendered non-conductive when there is a pulsating direct current at the output of said filter circuit and will be conductive when there is no pulsating direct current, and relay means actuated by the voltage drop across said cathode load resistor for keying the transmitter, said relay means including a vacuum tube having a control grid circuit and a plate circuit, the voltage drop across said cathode load resistor being applied to said grid circuit, a load resistor connected in said plate circuit, a potential supply for said plate circuit, the positive terminal of said potential supply being grounded and the voltage drop across said plate load resistor connected to the grid circuit of the transmitter keying stage.

FRED M. BERRY. WILLIAM R. LEWIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,695,042 Fearing Dec. 11, 1928 1,840,140 Vance Jan. 5, 1932 

